1. Field of the Invention
The present invention relates to a data communication apparatus used in communication using an optical fiber as a data transmission medium.
2. Description of the Prior Art
The IEEE Std. 1394-1995 (hereinafter IEEE1394) has been receiving widespread attention as a data communication method between digital home information appliances. As a complement to IEEE1394, the IEEE Std. 1394a-2000 (hereinafter IEEE1394a) aims at higher communication efficiency and lower power consumption, and a draft standard called p1394b tackles faster data rates over longer distances.
Tone Signals and Data Signals
According to p1394b, which is an example of optical two-way communication (full-duplex communication) using two optical fibers, connection between apparatuses is established by exchanging tone signals, and, after the establishment of connection, a data signal starts being transmitted when a request for communication occurs in one of the apparatuses. On the other hand, according to IEEE1394a, when no communication with a partner apparatus takes place after the establishment of connection, a suspended state (a connection-established state) is maintained by canceling the request for data communication to achieve low power consumption.
Detection of Disconnection Based on SD Signal
According to the standard p1394b, a binary signal called SD (signal detect) is used to detect disconnection between apparatuses. The SD signal is a signal that remains active while a tone signal is being oscillated and while a normal data signal is being received. When apparatuses are disconnected from each other as a result of physical disconnection or power-down, the SD signal becomes non-active, and this permits easy detection of disconnection.
Detection of Start of Communication Based on SD Signal
Moreover, when a transmitted signal is switched from a tone signal to a continuous data communication signal, the SD signal becomes continuously active. By detecting this, the receiving side can easily detect the starting of data communication by the communication partner.
Use of a Single Optical Fiber
While, as described above, p1394b is optical two-way communication using two optical fibers, there is observed a trend toward the realization of optical two-way communication (single-fiber full-duplex communication) using a single optical fiber for cost reduction and space saving.
Reason for Difficulty in Detecting Disconnection Based on SD Signal
However, in optical two-way communication using a single optical fiber, a light-emitter cannot be optically separated from a light-receiver, and therefore the light receiver of one apparatus (hereinafter the home apparatus) receives not only the light (partner light) transmitted from another apparatus (hereinafter the partner apparatus) but also part (stray light) of the light transmitted from the light-emitter of the home apparatus itself toward the partner apparatus. As a result, even when the two apparatuses are disconnected from each other, the stray light so received may keep the SD signal active. This occasionally makes it impossible to detect disconnection on the basis of the value of the SD signal alone.
Reason for Difficulty in Detecting Start of Communication Based on SD Signal
Moreover, when the home apparatus starts data communication first, the received stray light may make the SD signal active. This occasionally makes it impossible to detect the starting of data communication by the partner apparatus thereafter on the basis of the value of the SD signal alone.
Reason for Difficulty in Maintaining Connection-Established State
Moreover, after a transition from a state in which two communication apparatuses are transferring data to a connection-established state resulting from simultaneous cancellation of requests for data transfer in both apparatuses, even though the apparatuses try to maintain the connection-established state by exchanging tone signals, if the home apparatus transmits a tone signal at almost the same time as the partner apparatus transmits a tone signal, the home apparatus cannot distinguish whether the received tone signal is from the partner apparatus or from the home apparatus itself. This makes it impossible to maintain the connection-established state, occasionally causing an unwanted transition to a disconnected state.
Reason for Difficulty in Shifting from Disconnected State to Connection-Established State
Moreover, during a transition from a disconnected state to a connection-established state resulting from recognition of the partner apparatus through exchange of tone signals, if the tone signals used have a fixed period, once the tone signal from the home apparatus overlaps with the tone signal from the partner apparatus, the stray light makes it impossible for the home apparatus to distinguish, by the method of detecting tone signals on the basis of the SD signal, whether the tone signal received is from the partner apparatus or from the home apparatus itself. This makes it impossible to detect the tone signal of the partner apparatus. As a result, no transition to a connection-established state ever takes place, with a disconnected state maintained for ever.